This invention relates to stereo blend circuits in FM stereo receivers and more particularly relates to such a blend circuit that blends and reduces stereo separation as a direct function of noise in the composite stereo signal.
Stereo blending in a conventional FM receiver is accomplished by reducing stereo separation inversely with the FM signal strength as detected in the IF or RF section of the receiver. One means of accomplishing this is indicated by the decoder 10 shown in FIG. 1. The 38 KHz restored sub-carrier signal and the output of the signal strength detector circuit at conductor 11 to be used as a blend-control voltage are applied to the two inputs respectively of a multiplier 14 that serves as a voltage controlled attenuator of the 38 KHz signal. The output of mixer 14 and the stereo composite signal at conductor 12 are mixed to demodulate the composite signal and especially to produce the demodulated L-R portion of the composite signal. That demodulated L-R portion signal is mixed with the non phase shifted composite signal in the summer 18 to produce the left audio signal, while the demodulated L-R portion signal is mixed with the 180.degree. phase shifted composite signal in adder 19 to produce the inverted right audio signal. Another inverter (not shown) may be added in the right audio or left audio outputs if wanted to prevent the occurrence of acoustic canceling, especially at low frequencies, by the right and left signal driven speakers.
Shortcomings of blending decoders of this type stem from the fact that the FM signal strength does not correlate well with the noise appearing in the stereo composite signal. Furthermore, objectionable distortion in the stereo audio signals resulting from receiving the FM radio signal from an FM broadcasting station via two or more paths, a condition commonly referred to as "multipath" reception, is not greatly ameliorated by conventional blending circuits.
Decoders that blend inversely with the FM signal strength are typically designed to fully blend to monaural when the signal to noise ratio falls below about 50 db at which it is considered that the signal strength is low enough for multipath conditions to become less noticeable. However, fully separated stereo receiver operation could have been enjoyed under most other (than multipath) conditions with signal to noise ratios between 50 and 40 db, pointing up the necessity of serious compromise in conventional decoders that blend inversely with signal strength.
In a car radio, multipath reception, varies as the car moves relative to the transmitting station and relative to the terrain containing hills, buildings and other objects that variously reflect the transmitted radio signal and create changing multiple paths to the receiver. Multipath reception is therefore particularly objectionable in a moving stereo FM receiver.
Engineers have presented conflicting theories and empirical data relating to mechanisms by which FM-receiver audible noise is generated. In a paper by Rucktenwald and Torick entitled FMX Mobile Reception, IEEE Transactions on Consumer Electronics, Vol. 34 No. 4, pp. 921-8, November 1988, it is shown that multipath noise is suitably diminished by automatically disabling the expander in an FMX receiver. But in a paper by Bose and Short entitled A Theoretical and Experimental Study of Noise and Distortion in the Reception of FM Signals, RLE Technical Report No. 540, M.I.T., January 1989, it is concluded that "FMX" reception is inherently more susceptible to multipath noise and that the best remedy is conventional FM reception with a stereo blending capability.
"FMX" and "FMX Stereo" are TRADEMARKS of Broadcast Technology Partners of Bloomfield Hills, Mich., which company has proposed the FMX transmission system in which the standard FM composite signal includes in addition to the usual L-R base band of the suppressed 38 KHz/0.degree. subcarrier, a L-R base band of the quad subcarrier, 38 KHz/90.degree.. The FMX receiver retrieves and expands both for presentation to the receiver dematrixer circuit. This system is described in the Torrick et al U.S. Pat. No. 4,485,483, issued Nov. 27, 1984.
It is therefore an object of this invention to provide an FM stereo blending decoder that is more responsive to radio noise in the composite signal and that substantially reduces audio signal distortion during fixed or varying multipath conditions.